1. Field of the Invention
The present invention pertains generally to tunable laser devices and methods, and more particularly to an external cavity laser apparatus which provides high tuning speeds and highly predictable, constant change in laser output wavelength with respect to time.
2. Description of the Background Art
Optical communication and optical data storage technologies increasingly rely on use of tunable semiconductor lasers. The output of semiconductor lasers can be often tuned over a range of output wavelengths by adjusting the angular configuration of an external laser cavity. Typical external laser cavity arrangements utilize a retroreflective dispersive device, such as a diffraction grating, together with a movable end mirror which can be adjusted in angular position. Common external laser cavity configurations include the xe2x80x9cLittrowxe2x80x9d arrangement, in which the retroreflective dispersive element itself serves as a resonator end minor, and the xe2x80x9cLittman-Metcalfxe2x80x9d arrangement, wherein the retroreflective dispersive element is positioned between the end mirrors of a xe2x80x9cfoldedxe2x80x9d resonator cavity. The end mirror and/or retroreflective dispersive element are varied in angle with respect to each other to control tuning or selection of desired laser output wavelengths.
The evolving uses for tunable solid state lasers has increasingly required precise systems and methods for optical frequency or wavelength control. In the field of photonics, and particularly in the filed of passive optical components, the optical characteristics of components must be tested for manufacturing, quality control and other purposes. These characteristics typically vary with the wavelength of the light input to the component, and tunable solid state lasers have been used for evaluating such optical characteristics throughout the wavelength ranges of the components.
An important deficiency of presently available tunable solid state lasers is the lack of smooth, constant, accurate tuning action to provide highly predictable tuning speeds and output wavelengths during tuning. Even relatively small amounts of wear or machining error in parts and/or low levels of vibration make accurate tuning difficult to achieve. The tuning inaccuracies of currently available solid state lasers increases the time and expense required for characterization of optical components, and can result in reduced component quality.
Various tuning mechanisms for external cavity diode lasers have been designed and implemented to improve tuning accuracy through control of external laser cavity configuration. Currently available tuning mechanisms, however, have not provided the needed increases in tuning accuracy. These tuning mechanisms also tend to be relatively complex and are subject to increased wear and require a high level of maintenance. Tuning mechanisms which have achieved some smoothness in tuning have done so at the expense of undesirable reductions in tuning speeds.
Corrective measures have also been implemented to overcome the low tuning accuracy of currently available solid state lasers. One such approach has been use of a high-precision wavelength meter control system together with the tunable laser. The wavelength meter typically comprises a high-finesse interferometer and a reference optical wavelength provided by an atomic or molecular emission or absorption feature. The tunable laser is stepped through a series of measurements, with the laser output wavelength being set or calibrated using the wavelength meter as a provider of feedback. This allows adjustment of the output wavelength of the tunable laser in precise amounts or increments to provide equally spaced, accurate measurement of the optical properties of components under test. Corrective measures of this type, however, are time intensive and result in considerable increase in the testing costs for optical components.
There is accordingly a need for an external cavity laser apparatus and method which provides highly accurate and predictable tuning speeds for tunable solid state lasers, which provides for rapid tuning, which provides output at precisely known wavelengths, which is of simple, wear-resistant construction, and which allows for quick and accurate wavelength-dependent characterization of optical components. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies found in the background art.
An object of the invention is to provide an external cavity laser apparatus and method which allows highly linear or highly predictable change in output wavelength.
Another object of the invention is to provide an external cavity laser apparatus and method which allows fast tuning speeds.
Another object of the invention is to provide an external cavity laser apparatus and method which operates with low tuning noise.
Another object of the invention is to provide an external cavity laser apparatus and method having a simple tuning mechanism which is easy to manufacture and implement.
Another object of the invention is to provide an external cavity laser apparatus and method having tuning mechanism which provides low friction, low wear and low-hysterysis tuning movement.
Another object of the invention is to provide an external cavity laser apparatus and method which delivers highly predictable output wavelengths without requiring complex corrective measures or control systems.
Another object of the invention is to provide a constant tuning speed micropositioning apparatus and method which allows quick and precise characterization of wavelength dependent properties of optical components.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing the preferred embodiment of the invention without placing limitations thereon.
The present invention is a high precision, high tuning speed external cavity laser apparatus and method for tunable solid state lasers and the like. The invention provides for fast, highly predictable tuning of laser output wavelength or frequency and allows quick and easy wavelength- or frequency- specific characterization of optical components.
In its most general terms, the invention comprises an external cavity laser apparatus and method for use that provides an output wavelength which varies linearly and/or with high predictability over time, and which can provide tuning speeds of greater than 100 nanometers per second. More specifically, the invention comprises a laser resonator cavity having first and second reflective elements, at least one of which is movable to provide a constant or highly predictable change in output wavelength with respect to time. A tuning assembly associated with the movable reflective element is structured, configured and positioned to positionally adjust the movable reflective element such that constant changes in output wavelength and high tuning speeds are provided.
In a first presently preferred embodiment of the invention, the tuning assembly comprises a cam body, a motor or like rotating drive source which rotatably drives the cam body, an actuator arm which is mechanically interfaced with the cam body, and a low friction cam interaction surface associated with the mechanical interface of the cam body and the actuator arm. The actuator arm is coupled to the movable reflective element of the resonator cavity, and rotational movement of the cam body is transferred to the movable reflective element via the actuator arm such that the movable reflective element undergoes positional adjustment responsive to movement of the cam body. The output wavelength can be tuned with high linearity and/or predictability with respect to the rotational speed of the drive motor.
By way of example, and not of limitation, the actuator arm preferably is pivotally mounted to a base or surface at a pivot point such that the optical element undergoes positional adjustment which is pivotal in nature with respect to the pivot point. A cam follower pad is positioned on the actuator arm and interacts directly with the cam body. The cam follower pad preferably comprises a low friction material or includes a low friction surface. The cam body likewise has a low friction surface. The low friction surfaces provide smooth, vibration-free mechanical interaction between the cam body and cam follower pad. The rotational drive source is preferably a low vibration brushless DC motor. The structure and configuration of the cam body can be varied to provide a particular type, speed and range of movement to the positionally adjustable reflective element.
The invention is particularly well suited for use with external cavity solid state lasers. In this regard, the second resonator end reflective element will generally comprise the reflective rear facet of a solid state laser, which is fixed in position with respect to the movable reflective element. A retroreflective dispersive element such as a diffraction grating is fixedly positioned between the solid state laser and the movable reflective element. The cam body is structured and configured to provide a constant, linear tuning speed to the external cavity laser in terms of wavelength per second. The radius of the cam body changes according to its eccentricity to provide a change in feedback angle in the actuator arm and attached movable reflective element, and hence provides a change in output wavelength.
In operation, the action of the drive motor causes the cam body to rotate. As the cam body rotates, the cam body is in contact with the actuator arm, and the eccentricity of the cam body results in movement of the actuator arm and attached resonator end reflective element about the pivot point of the actuator arm. The pivotal movement provides both rotational and translational motion to the resonator end reflective element and results in change in length of the resonator cavity and tuning of the laser output wavelength or frequency. The shape of the cam body can be tailored to provide a particular desired rate in change in output wavelength with respect to motor rotational position xcfx89. Particularly, the cam body may be structured and configured such that dxcfx89/dt=dxcex/dt=a constant value.
The high predictability of the laser output wavelength with respect to time as provided by the invention reduces the level of electronic feedback necessary for control of the output wavelength. Preferably, an optical encoder is associated with the brushless DC motor. The encoder is interfaced with a servo control system, and a linear amplifier is used to control the motor according to feedback from the encoder. The use of a linear amplifier, rather than a conventional pulse width modulated amplifier, provides for reduced noise while tuning the output wavelength.
In a second preferred embodiment of the invention, the tuning mechanism comprises a weighted pendulum and a pivoting actuator arm coupled thereto at one end of the actuator arm. The movable reflective element is coupled to the other end of the actuator arm. The pendulum is moved or pushed to provide an oscillating, pendular motion to the pendulum and actuator arm. As the pendulum actuator arm moves, the attached reflective element undergoes an oscillating movement which provides highly predictable tuning of the output from the laser.
In a third preferred embodiment of the invention, the tuning assembly comprises a pivoting actuator arm directly coupled to a low vibration rotational drive source. The movable reflective element is mounted on the actuator arm and undergoes positional adjustment as the actuator arm moves pivotally with respect to the drive source.
In a fourth preferred embodiment of the invention, the tuning assembly comprises a pivoting actuator arm associated with an arcuate voice coil. The voice coil includes an arcuate actuator magnet coupled to the actuator arm, and arcuate wire coils around the actuator magnet which are fixed externally with respect to the actuator arm and actuator magnet. Passage of current through the wire coils results in movement of the actuator magnet, actuator arm and attached reflective element. The angle of arc of the voice coil is structured and configured according to the length of the actuator arm and position of the pivot point of the actuator arm.
In a fifth preferred embodiment of the invention, the tuning assembly comprises a pivoting actuator arm with a first magnet coupled thereon, and a second magnet coupled to an external drive source. The first and second magnets are positioned proximate to each other to exert a mutually opposing magnetic force against each other such that movement of the second magnet by the external drive source results in movement of the first magnet together with the attached actuator arm and reflective element.
The use of the preferred tuning assembly embodiments as provided by the invention allows highly predictable laser tuning output and fast tuning speeds. The preferred tuning mechanisms are easy and inexpensive to manufacture and implement. The absence of screws, gears and like moving parts in tuning mechanisms of the invention provides for low friction, low wear and low-hysterysis movement for predictable, accurate, low noise tuning without use of a wavelength meter or other corrective measures or complex control systems. The high predictability and fast tuning speed of the laser output wavelength provided by the invention allows quick and precise characterization of wavelength dependent properties of optical components.